1. Field of the Invention
The present invention relates to apparatus for facilitating the use of a fiber optic sensor array. More particularly, this invention pertains to a connector for simultaneously completing a plurality of reliable low-loss optical interconnections.
2. Description of the Prior Art
SONAR-based systems for detecting underwater hazards employ pressure transducers of the hydrophone type for generating signals indicative of the presence and locations of underwater objects. In an active system, such objects are sensed by means of acoustic signals that are generated and transmitted from a mother ship. The reflected acoustic signals, when processed, enable one to detect and ascertain the signature, position, velocity and other features of underwater objects such as submarines. In passive systems, acoustic signals are generated by and radiated from the submerged object.
Fiber optic (visa vis electrical) technology offers a number of advantages in hydrophone design. Since fiber optic systems are electrically passive, the risks associated with exposure of electrical elements to water are avoided. The use of all-optical telemetry data systems is facilitated as the sensor is itself an optical device. In a completely optical detection and telemetry system, extraneous electromagnetic noise is eliminated. Other advantages include an increase in system bandwidth and considerable weight reduction.
The optical signals generated by the fiber optic hydrophone are transmitted along optical fibers. The thread-like optical fiber presents a relatively flimsy element requiring mating with a mechanical termination element to permit accurate positioning of the fiber end for optically coupling the signal therein to another fiber when required. A useful optical fiber termination is the well-known optical ferrule. One type, the ceramic ferrule, comprises a shell (e.g. zirconia) with an adhesive interior for holding the fiber. Unfortunately, differences in the coefficients of thermal expansion of the zirconia and the adhesive can result in adhesive creep in which the adhesive flows, degrading performance. An improvement in this regard is the tungsten carbide ferrule comprising a shell of tungsten carbide and an Ni/Ag filler material that surrounds the terminal end of the fiber. The coefficient of thermal expansion of the shell and the filler material do not differ significantly from one another, eliminating the problems associated with significant creep or flow. Examples of commercially available ferrules of the tungsten carbide type are those marketed by Diamond SA of Losone, Switzerland under designations FC-PC, ST-PC and DIN.
In a common application, an optical SONAR system employs a predetermined submergeable array comprising a plurality of acoustic hydrophones to collect a spatial distribution of acoustic information for analysis and signal processing purposes. The fiber optic hydrophone array is usually housed within a hose-like element that is fixed to the end of a towing cable that comprises an arrangement of optical fibers within a protected outer jacket. An example of such a cable is disclosed in U.S. Pat. No. 4,952,012 of Stamnitz entitled "Electro-Opto-Mechanical Cable For Fiber Optic Transmission Systems." The processing of data from an array is well-known and discussed, for example, by A. Dandridge et al. in the article "Multiplexing of Interferometric Sensors Using Phase Carrier Techniques," Journal of Lightwave Technology, Vol. LT-5, No. 7 (July 1987) at pages 947-952.
While the fiber optic towed array is a recognized element of an optical SONAR system, its realization in practice is complicated by a number of serious mechanical and optical design considerations. Towed fiber optic hydrophone arrays can include a multiplicity of array modules, each comprising a hose-like covering that encloses a serial hydrophone arrangement. The modules are, in turn, coupled to one another and to a tow cable. Optical connections must be made through the hydrophones and to a mother ship. In the case of an array that includes a large plurality of hydrophones, a correspondingly large number of reliable optical connections must be made and maintained simultaneously to assure successful operation.
The maintenance of multiple reliable optical contacts in the operational environment is complex. Optical fiber connections, unlike electrical, demand precise optical alignment of the members and can be significantly degraded by the presence of environmentally induced forces. The failure to obtain precise alignment of fiber terminations can contribute significantly to optical signal loss. This is in contrast to electrical connections which essentially require only that contact be maintained.
Due to the frequent utilization of acoustic sensing systems for military applications, hydrophone systems must meet stringent military specifications that often exceed those of the commercial world. Such uses impose very low insertion loss requirements (typically less than a few tenths of a decibel) that place even greater emphasis upon the maintenance of precision alignment. In addition to accurate alignment, low loss operation also requires the maintenance of good optical contact between conductors. This is particularly difficult when one deals with the single mode optical fiber that is generally employed in fiber optic sensor arrays. The core diameter of such fiber is typically much smaller than that of "ordinary" multimode fiber. Connections of such fibers are sensitive to not only axial but also to rotational misalignment. Either type of misalignment will introduce noise due to poor coupling, optical interference and/or retroflection.